Turbines generally include multiple stages of rotating blades, also known as turbine buckets, circumferentially located around a rotor. Each turbine bucket generally includes a root portion that connects to the rotor, an airfoil portion that extends radially from the root portion, and a shroud portion that connects to the airfoil portion at the outer radial end of the turbine bucket. A working fluid flowing through the turbine causes the turbine buckets and rotor to rotate to produce work. A bearing surface on each side of the shroud portion allows adjacent turbine buckets to interlock at the outer radial extremity. This radial interlocking of turbine buckets prevents the turbine buckets from vibrating, thereby reducing the stresses imparted on the turbine buckets during operation.
Turbine buckets are often cast from molten metal, and directionally solidified turbine bucket castings are commonly used to improve the efficiency and performance of turbine buckets beyond that previously available with conventional castings. In directional solidification castings, molten metal is supplied to a vertically oriented turbine bucket mold having a cavity for the root, airfoil, and shroud portions of the turbine bucket. A chill plate located at the lower end of the mold, for example adjacent to the root portion of the mold, rapidly removes heat and creates a vertical temperature gradient along the mold. The vertical temperature gradient promotes the growth of directionally solidified grains normal to the chill plate from the root portion, through the airfoil portion, and ending at the shroud portion. However, angles, curves, indentations, and other changes in the mold shape alter the direction of the grain growth, resulting in the development of multiple directionally solidified grains as the molten metal cools. The multiple directionally solidified grains converge at the vertically highest portion of the mold, for example the shroud portion of the turbine bucket. The convergence of the multiple directionally solidified grains results in multiple grain boundaries in the shroud portion of the turbine bucket.
The multiple grain boundaries in the shroud portion of the turbine bucket are characterized by high interfacial energy, a relatively weak bond, and a susceptible fracture path. As a result, the shroud portion may be more susceptible to an earlier onset of corrosion, creep, fatigue failure, cracking, and other failure mechanisms. This may be of particular concern at the bearing surfaces of the shroud portion that experience additional stress and fatigue cycles during normal operations. Therefore, a system and method for reducing grain boundaries in the shroud portion of turbine buckets may be useful.